Field of the Invention
The present invention relates to a photoacoustic image generation apparatus and method, and particularly to a photoacoustic image generation apparatus and method that detects, after light has been output to a subject to be examined, a photoacoustic wave induced in the subject to be examined, and generates a photoacoustic image.
Description of the Related Art
Ultrasonography is known as one of image examination methods that can perform non-invasive examination to find the condition of the inside of a living body. In ultrasonography, an ultrasonic probe that can send and receive ultrasounds is used. When an ultrasound is sent from the ultrasonic probe to the subject to be examined (living body), the ultrasound travels in the living body and is reflected at a tissue interface. The reflected ultrasound is received by the ultrasonic probe. It is possible to generate an image representing the state of the inside of the subject to be examined by calculating a distance based on the length of time till the reflected ultrasound returns to the ultrasonic probe.
Further, photoacoustic imaging, which images the inside of a living body by utilizing photoacoustic effects, is known. Generally, in photoacoustic imaging, pulsed laser light, such as a laser pulse, is output to the inside of the living body. In the inside of the living body, living tissue absorbs energy of the pulsed laser light, and an ultrasound (photoacoustic signals) is induced by adiabatic expansion caused by the energy. The photoacoustic signals are detected by an ultrasonic probe or the like, and a photoacoustic image is constructed based on the detected signals. Accordingly, it is possible to visualize the inside of the living body based on the photoacoustic signals.
Here, in generation of an ultrasonic image and a photoacoustic image, image reconstruction methods are roughly classifiable into two kinds of method, i.e., a time domain method and a Fourier domain method (Fourier transform method). The time domain method utilizes the feature that a sonic wave is a spherical wave, and draws candidates of a sonic wave source in real space. Typical examples of the time domain method are a DnS (Delay and Sum) method, and a CBP (Circular Back Projection) method, and an HTA (Hough Transform Algorithm) method.
Meanwhile, image reconstruction by the Fourier transform method (FTA (Fourier Transform Analysis) method) utilizes the feature that measured information in time space is able to be transformed to information in real space by using Fourier transform and dispersion relations of sonic waves. Specifically, first, a spectrum in time scale is obtained by performing forward Fourier transform on a time series of measured sonic waves. Then, transformation from time scale to space scale is performed based on the dispersion relations of sonic waves (ω=ck, ω: temporal frequency, and k: spatial frequency), and after then, inverse Fourier transform is performed. Accordingly, the spatial distribution of sonic wave sources is obtained as an image.
Japanese Unexamined Patent Publication No. 2010-035806 (Patent Document 1) discloses generation of a photoacoustic image by utilizing image reconstruction by the time domain method and image reconstruction by the Fourier transform method. In the image reconstructed by using the time domain method and the image reconstructed by using the Fourier transform method, artifacts appear in different manners from each other. Patent Document 1 utilizes such characteristics, and obtains a photoacoustic image in which artifacts are suppressed by combining the images reconstructed by using the two different methods.